1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for redistributing oil to retaining pads within a bearing device.
2. Discussion of the Background
Turbo machinery is evolving and the latest technology in this field is making use of high-speed bearings. In many areas of bearing design, the design of the rotor-bearing system directly influences the performance of the machine. Traditional designs have provided bearings with rolling elements, i.e., pads or shoes that may pivot around a retaining head while supporting a rotor. However, at high speeds and/or high pressures, the load capacities and stiffness limits of the rolling elements are exceeded and thus, the performance and life expectancy of the machinery is reduced. For example, at peripheral speeds above the typical speed for a traditional turbo machinery, ball-bearings placed at the ends of a rotating shaft to accommodate bearing speed limits may lead to super critical operation (i.e., operating above critical speed), which may result in an unstable rotor subject to destructive and unpreventable subsynchronous whirl and to large radial deflections.
While conventional applications of turbo machinery employ a traditional peripheral speed, it appears that a machine that can operate at higher speeds would improve power consumption and heat distribution in pads, among other advantages. However, these higher-than-normal speeds may contribute to other problems that are discussed next.
In order to reduce friction between a rotor and a bearing, oil may be introduced to separate the two components of the machine. In the art, this system is known as a journal bearing. The shaft and bearing are generally both simple polished cylinders with lubricant filling a gap between the shaft end and the shoes of the bearing. Rather than the lubricant just “reducing friction” between the surfaces of the shaft and the shoes, letting one slide more easily against the other, the lubricant is thick enough that, once rotating, the surfaces do not come in contact at all. If oil is used, it is generally fed into a hole in the bearing under pressure, as is done for loaded bearings.
Such an example is shown in FIG. 1, which is an illustration of FIG. 1 of U.S. Pat. No. 6,361,215, the entire content of which is incorporated herein by reference. FIG. 1 shows the journal bearing 10 enclosing a shaft 12 that rotates in a direction as shown by arrow 14. Journal bearing 10 includes five pads 16 that are retained in place by a ring 18. Each pad 16 includes a pad support 20 inserted into a recess region 22 of the pad 16. The pad support 20 is connected to an insert 24 that is fixed to the ring 18. Each pad support 20 and recess region 22 have cooperating spherical surfaces to allow the pad 16 to pivot freely in any direction to conform to the surface of the shaft 12 as it rotates. In addition, the journal bearing 10 has oil supply devices 26 regularly formed inside the ring 18 for supplying the oil between pads 16 and shaft 12.
However, when shaft 12 rotates relative to the pads 16 above a certain speed, oil starvation is one of the problems found in traditional turbo machinery. This problem is exacerbated when the peripheral shaft speed is increased. Oil starvation is the lack of enough oil for the rotating shaft and/or pads such that the oil film between the shaft and the pads is interrupted, which may lead to high friction between the shaft and pads, leading to high temperature and subsequent damage.
In response to this problem, the above noted U.S. Pat. No. 6,361,215 proposes a solution as shown in FIG. 2, which corresponds to FIG. 3 of the patent. FIG. 2 shows a surface 16a of the pad 16 having a leading edge 28a and a trailing edge 28b. The pad 16 has the oil supply devices 26 in flow communication with a groove 30. Thus, the oil is initially provided on the surface 16a in groove 30. From there, the oil is taken by the rotor (while rotating along direction 14) towards the inside of the surface 16a of pad 16. However, as explained by U.S. Pat. No. 6,361,215 at column 5, lines 43-47, the oil is forced from a center of surface 16a towards edges 32a and 32b of the pad 16. To improve an oil distribution on surface 16a, U.S. Pat. No. 6,361,215 proposes adding grooves 34a and 34b on sides of the pad 16, to capture the oil moving from the center of the pad towards edges 32a and 32b for redistributing that oil. Thus, grooves 34a and 34b of U.S. Pat. No. 6,361,215 have two regions, first regions 36a and 36b that are parallel to edges 32a and 32b and second regions 38a and 38b that are angled to facilitate the oil distribution from the side towards the center of pad 16.
However, this method only provides oil redistribution close to the trailing edge 28b, where the second regions 38a and 38b of grooves 34a and 34b are located, and the redistribution takes place from the edges of pad 16 towards the center of pad 16. Thus, the oil redistribution is limited by the geometry of the second regions 38a and 38b. FIG. 3 shows a pressure distribution of the oil across a width W of the pad 16, which illustrates why the oil from the center of surface 16a in FIG. 2 is forced to move towards edges 32a and 32b by the rotation of shaft 12.
Another problem that may appear in traditional turbo machinery is cavitation. Cavitation is the formation of vapor bubbles of a flowing liquid (oil for example) in a region where the pressure of the liquid falls below its vapor pressure. As the rotor rotates at high speed, the pressure of the oil may fall below its vapor pressure, leading to cavitation and formation of shock waves. Since the shock waves formed by cavitation are strong enough to significantly damage moving parts, cavitation is usually an undesirable phenomenon.
As the peripheral shaft speed of new applications requires speeds in excess of predetermined speed, the above summarized problems have to be addressed and solved in order for these applications to function appropriately. While the above problems have been discussed in the context of journal bearings, these problems are found in other bearings in which the shaft rotates relative to the pads at high speeds.
Accordingly, it would be desirable to provide devices, systems and methods that avoid the afore-described problems and drawbacks as well as others understood by those of ordinary skill after consideration of the subject matter disclosed below.